With the remarkable development of silica optical fibers, a light transmission systems using optical fibers have lately been put to practical use. Plastic optical fibers show a relatively large loss in light transmission (about 1 dB/m), compared with that of silica optical fibers occupying the mainstream of the present light communication developments. However, the plastic optical fiber is very flexible and relatively soft, may have a large aperture, and can easily be handled, and thus it may be expected that this plastic optical fiber will be applied to such short distance communication as in an automobile, office and factory automations, and the like.
Polymethacrylate resin or polystyrene resin having an improved optical property is used for the present plastic optical fibers. However, these resins have a relatively low thermal deformation temperature and thus it has been desired to improve its thermal resistance in various fields, including the automobile industry. Accordingly, an investigation of thermal resistant plastic optical fibers has lately been carried out.
Conventional plastic optical fibers have a core comprising acrylic, styrene or silicone thermosetting resin having an improved thermal resistance, and a cladding comprising fluororesin and the like having improved thermal resistance and a lower refractive index than that of the core.
In such a thermal resistant, plastic optical fiber, the resin precursor (monomer) used for the core is liquid at room temperature, and thus it is difficult to form a fibrous core by itself from this liquid monomer; as the temperature to process a fluororesin material is higher than 200.degree.C., it is difficult to extrude the fluororesin material around the core for its cladding. In order to solve these problems, Japanese Laid Open Patent Publn. No. 45502/82,for example, discloses the following method of making a plastic optical fiber. This method comprises extruding a resin material to form a hollow clad tube, injecting a liquid monomer material used for a core from a tank into the resulting clad tube, sealing off the leading end of the clad tube, and gradually passing the tube containing the monomer under pressure through a hot water bath to cure the monomer from its leading portion to its rear end to provide a plastic optical fiber without separation of the interface between the core and the clad tube.
Regarding the characteristics of the optical fiber, it is the most important to minimize the transmission loss of light.
Low-loss, visible light is used in the transmission of light by means of the plastic optical fiber. However, this visible light will result in the following problem: as the two layered types of plastic optical fiber described above has a transparent clad layer, external visible light can intrude into the transparent clad layer and this can result in transmission loss.
In order to solve this problem, a structure to prevent the intrusion of the external visible light by coloring such a clad layer has been proposed. However, a portion of the transmitted light is absorbed by the colored clad layer, causing transmission loss.
As described above, such a two layered type of plastic optical fiber comprising a core and a cladding has the problem of such transmission loss even if transparent or a colored layer is used as the clad layer. As one of the means to solve these both problems, the method for extruding a colored sheath around the transparent clad layer to form a plastic optical fiber has been proposed so as to prevent the intrusion of the external light and the absorption of light by the transparent clad layer.
However, the plastic optical fiber having such a sheath around the clad layer has various problems and is very difficult to put to practical use.
That is, as described above, when the liquid monomer used for a core is polymerized within the clad tube, it is required to supply the liquid monomer to the clad tube under pressure by considering the volume contraction of the liquid monomer upon polymerization and thus, it is impossible to make a very thin clad tube and the further step of sheathing would provide an optical fiber having an oversized outside diameter. This oversized diameter is undesirable with respect to the characteristics and use of the resulting optical fibers.
In addition, such a sheath around the preformed plastic optical fiber results in a thermal strain between the core and clad layer, and between the clad layer and the sheath due to thermal hysteresis, and thus a high transmission loss (about 2-3 dB/m).
When the sheath is first extruded around the clad tube to form a sheathed clad tube, then the monomer for the core is injected into the sheathed clad tube and the injected monomer is heated for polymerization, the resulting product has poor adhesion between the clad tube and the sheath and thus a low transmission efficiency.
As described above, in either case where the resin material for the core is polymerized using the product obtained by sheathing of the preformed plastic optical fiber of sheathing of the clad tube, the resulting products have undesirable transmission characteristics. Furthermore, when a thermodynamic cycle is applied to these plastic optical fibers, both of them show the so-called "shrink back" (this term means that the clad layer protrudes from the sheath due to the poor adhesion between them), and thus an undesired transmission loss. The plastic optical fiber is generally used with a connector at its end portion and thus its high transmission loss is caused by this "shrink back".